A thin disk laser is a laser whose laser active medium is a thin disk element. Thin disk lasers generally comprise a thin disk element, usually crystal, and an optical resonator cavity, which includes end mirrors and a partially reflective mirror, used as the output coupler. Typically, the thin disk element is approximately a few tenths of a millimeter to a few millimeters in thickness. One flat side of the disk element has a highly reflective (HR) layer, while the opposite flat side has an anti-reflectivity (AR) layer.
To achieve population inversion, the thin disk element is optically pumped with light of an appropriate wavelength, which is incident on the flat surface of the side of the disk element having the anti-reflectivity layer. As the disk element is very thin, a relatively low percentage of pump light is absorbed after one pass. Therefore, unabsorbed pump light is repeatedly reflected back towards the thin disk by a series of reflective optical elements, thereby achieving sufficient pump light absorption. Emitted laser light resonates within an optical resonator cavity, thereby amplifying the laser light, some of which is passed through a partially reflective output coupler. As both laser light and pump light must be reflected by the thin disk element, the highly reflective layer on the thin disk element must be reflective with respect to both the pump light wavelength and the laser light wavelength. The anti-reflectivity layer likewise must have minimal reflectivity at both the pump and laser wavelength.
Laser resonator cavities can be constructed in many ways, and generally comprise a series of optically reflective elements aligned in such a way as to allow multiple passes of laser light and pump light over the thin disk element. Examples of laser optical cavity resonators can be seen in the following patents: U.S. Pat. Nos. 6,347,109 and 7,308,014.
Typically, cooling is required for the thin disk element. It is preferable to cool from the side having the high-reflectivity coating (the bottom or back side), since laser light and pump light are both incident on the side having the anti-reflectivity coating. Cooling from the bottom side allows use of a cooling structure with no optical requirements.
A very effective method of cooling the high reflectivity surface is liquid cooling. However, liquid cooling applied directly against the HR coating tends to degrade the reflectivity performance of the high reflectivity coating as the liquid coolant physically degrades the thin, sensitive material of the HR layer. Some sort of barrier material is therefore necessary to prevent degradation of the HR layer.
One prior attempt at a solution to the problem of degradation of the high reflectivity coating during liquid cooling is the use of hard durable coatings deposited by ion-beam sputtering (IBS). However, our tests have shown that this method is insufficient to properly protect the HR coating against degradation, as measurable degradation was noticed after only approximately 40 hours of cooling.
Another prior attempt at a solution to the problem of degradation of the high reflectivity coating during liquid cooling is to use a thick protective material such as synthetic diamond made by chemical vapor deposition (CVD), glued to the HR stack. This provides some protection for the HR coating, with excellent thermal conductivity, but there is a severe mismatch in the coefficient of thermal expansion between the diamond, the YAG active laser material, and the HR coating. This mismatch prevents the CVD diamond solution from use over a large range of coolant temperatures, and may prevent, for example, its use with a cryogenic coolant.
Thus there is a need for long-term protection against degradation caused by liquid cooling of the bottom side of thin disk elements, while providing good thermal conductivity for effective cooling, as well as good coefficient of thermal expansion (CTE) matching, thereby allowing operation over a large coolant temperature range.